This invention relates to dielectric fluids for use in transformers. In particular, it relates to dielectric isoparaffinic based tranformer fluids.
Conventional transformer oils are typically manufactured from a vacuum g oil fraction derived from naphthenic crudes and in particular light naphthenic distillates. Although transformer oils made from naphthenic crudes perform adequately they are inherently deficient in certain respects. For example, naphthenics are compositionally rich in potentially toxic aromatics and as result there is a desire for compositionally cleaner transformer fluids. At the same time, some of the nathphenic crudes, which are especially suitable for transformer oil manufacture, are being to dwindle. As a result there is a desire to supplement the transformer oil pool with other sources.
Attempts have been made to develop paraffinic-based transformer oils. However, none has been successfully commercialized as they have been deficient in several respects Specifically, such paraffinic based transformer oils have inherently poor low temperature viscometric properties. Also they do not exhibit negative gassing performance as determined by ASTM D2300B, which is considered by the electrical industry to be an important feature Consequently, naphthenic based transformer oils which have inherent pour points of  less than xe2x88x9240xc2x0 C. and exhibit negative gassing are sill preferred by the electrical industry. Negative gassing performance is important since in the event that hydrogen is evolved due to electrical stress the fluid tends to absorb the evolved hydrogen thus reducing the chances of an explosion.
U.S. Pat. No. 5,167,847 to Olavesen et al. discloses a transformer oil derived from a hydrocracked, solvent dewaxed base oil having a pour point of about xe2x88x9221xc2x0 C. This is achieved by the addition of antioxidant and 0.01 to 2.0 wt. % of a pour point depressant. The transformer oil has a positive gassing tendency,
U.S. Pat. No. 4,124,489 to Reid discloses a transformer oil from waxy crudes by double solvent extracting a raw, untreated, light distillate fraction from a waxy crude oil to produce a second, wax-containing extract. The second extract oil is mildly cracked by hydrotreating, also reducing the sulfur content and improving the viscosity, oxidation and color stability thereof. The hydrotreated oil is then distilled to produce a transformer oil feedstock of relatively low wax content as a heart cut fraction having a 5 to 95 LV % boiling range between about 595xc2x0 F. and about 750xc2x0 F. The transformer oil feedstock may then be dewaxed to produce a finished transformer oil.
G. L. Goedde et al (U.S. Pat. No. 5,766,517) discloses transformer oils made from synthetics such as poly alpha olefins (PAOs) or blends of synthetics and certain aromatic and olefinic additives which are added to yield negative gassing products. The transformer oils can be made by blending a PAO, for example made by the oligomerization of decene with an aromatic stream. One drawback to this approach is that, because of the cost of the PAO, the end product is very expensive in comparison with traditional transformer oils.
Sapienza (U.S. Pat. No. 5,912,215) discloses the manufacture of a food grade transformer oil based on blending a synthetic poly alpha olefin or a technical white oil with 10 to 70% of an unsaturated hydrocarbon, such as unsaturated poly alpha olefin decene dimer or polyisobutene. The drawbacks to this approach are high cost in the case of PAO and poor low temperature performance in the case of technical white oils. Additionally, olefins are oxidatively very unstable and as a result pose a potential oxidative and thermal instability problem in the event that the antioxidant which, are part of any transformer oil formulation, is depleted.
U.S. Pat. No. 5,949,017 to Oommen et al. discloses a transformer fluid derived from high oleic acid triglyceride compositions that include fatty acid components of at least 75% oleic acid, less than 10% di-unsaturated fatty acid component; less than 3% tri-unsaturated fatty acid component; and less than 8% saturated fatty add component. Although the fluid is biodegradable it is relatively expensive in comparison with conventional naphthenic transformer oils. Additionally, as with any ester there is the concern of hydrolytic stability in case the oil is inadvertently exposed to water and high temperatures.
Commandeur et al. (U.S. Pat. No. 5,545,355), teaches how to make a transformer fluid for low temperature applications. The fluids include a mixture of benzyltoluene and (methylbenzyl)xylene isomers, notably a mixture of benzyltoluene/dibenzyltoluene isomers with (methylbenzyl)xylene/di(methylbenzyl)xylene isomers.
Shubkin et al (U.S. Pat. No. 5,250,750) discloses an electrical insulating fluid based on compositions containing up to 25 weight percent of one or more oil additives and a 1-octene and/or 1-decene dimer and/or a 1-octene and 1-decene co-dimer oil having improved low temperature properties. The fluid contains less than about 25 weight percent of 7-methylpentadecene, 9-methylnonadecene and, 7- and 9-methylheptadecene isomers, respectively.
Sato et al. (U.S. Pat. No. 5,017,733) describes an electrical insulating oil composition which has 45% by weight or more of at least 2 members selected from the group consisting of (a) m-ethylbiphenyl, (b) p-ethylbiphenyl, (b) p-ethylbiphenyl, (c) o-benzyltoluene, (d) m-benzyltoluene, (e) p-benzyltoluene, and (f) 1,1-diphenylethane. The remainder of non-condensed bicyclic aromatic hydrocarbons have no more than 17 carbon atoms.
Accordingly, there still exists a need for a paraffinic based transformer oil which exhibits acceptable low temperature pour points and negative gassing properties.
It has now been discovered that it is possible to make a cost effective dielectric fluid from isoparaffinic oils, and preferably from hydroisomerized paraffinic oils. The formed dielectric fluids exhibit low temperature performance and oxidation stability which is equal to, or superior to, that observed with naphthenics. Transformer oils based on these types of hydroisomerized fluids exhibit excellent biodegradability characteristics.
Additionally, the present invention is directed to an isoparaffinic based transformer oil, which exhibits negative gassing properties and has a low temperature pour point. In one aspect, the present invention relates to a transformer oil comprising a base oil and a hydrogen donor. In a preferred embodiment, the transformer oil base stock is prepared from a isomerized or isoparaffinic oil, which is obtained from a sequential hydrocracking/hydroisomerization/hydrogenation process. The hydrogen donor may be any compound which contains labile hydrogen, for example a partially saturated aromatic compound such as tetrahydronaphthalene, alkyl substituted tetrahydronaphthalene compounds or alkylated benzenes. The transformer oil may also include one or more anti-oxidant compounds.
In another aspect, the present invention relates to a process for reducing the volume of hydrogen gas evolved from a transformer oil, the process comprising adding at least one hydrogen donor to the transformer oil. In a preferred aspect, the compound is tetrahydronaphthalene, alkylated tetrahydronaphthalenes and alkylated benzenes, and from about 0.1 to about 10 wt % is added to the transformer oil, based on the weight of the transformer oil.